For many years attempts have been made to improve the sailing efficiency of yachts in order to increase their sailing speed while retaining safety and ease of handling. Catamarans have become extremely popular due to their stability and high speed sailing ability. This high speed potential is achieved by the use of narrow hulls which do not have a displacement speed limitation and by the relatively high power to weight ratio which may be achieved because of the inherent stability of the catamaran configuration.
The power to drive such vessels is derived from sails set fore and aft. It has been realized for many years that the force produced by the sails includes a driving components which moves the boat forward and a lateral component which normally causes the boat to heel and move sideways and that the efficiency of a yacht may be increased by inclining the sails so as to utilize the lateral component for producing lift and for reducing the sideways component.
To date however the only commercially successful yachts to utilize sails inclined to produce lift have been sailboards. Inclined sails have also been used on high speed craft such as proas which are reversible twin hulled boats which maintain their respective hulls always in a selected windward/leeward configuration. These arrangements have been used on specialized racing craft only and have not been suitable for general purpose yachts due to practical disadvantages.
Conventional proas have a main or heavy hull and an outrigger or light hull whereby the centre of gravity of the yacht is offset towards either the windward or leeward hull of the craft. In the first mentioned configuration the relatively heavy windward hull gives increases stability provided the vessel is maintained with its heavy hull to windward. If the hull is positioned to windward the ability of the vessel to stand up to the sail force is greatly reduced making a capsize probable. In the second mentioned type of proas the overturning moment created by the sail force is resisted by varying the weight of the windward hull, generally either by placing crew on the windward outrigger or by ballasting the latter with water.